1. Field of the Invention
The present invention relates to wireless communication systems, and more specifically, to a wireless communication system which includes a plurality of cells and is suited for suppressing radio interference from adjacent base stations.
2. Description of the Related Art
In a CDMA cellular system, the base stations of adjacent cells use radio waves in the same frequency band. Radio interference can occur between those cells. The degree of interference depends on the propagation environment and the positional relationship between the base stations. Interference provides an insufficient signal-to-interference (S/I) ratio, and high-speed data transmission becomes impossible sometimes. Accordingly, stable communication must be established by increasing the processing gain, using a modulation system having a high degree of noise immunity, such as BPSK, or the like. A sufficiently high S/I ratio allows the processing gain to be decreased or allows the use of a multi-level modulation system having a low degree of noise immunity, such as 16QAM, enabling high-speed data transmission. Thus, interference from another cell greatly affects the transmission data rate in the CDMA system.
The cdma2000 1xEV-DO system will be described below as an example of the CDMA system. The system performs best-effort control, and detailed specifications of the system can be obtained from non-patent document 1. The best-effort system does not serve all mobile stations uniformly but attempts to improve the radio channel efficiency, giving a high priority to a mobile station having a good channel quality. Each mobile station determines parameters of communication channel modulation and encoding in consideration of an observed degree of interference noise and reports the parameters to the base station. The base station receives the parameters and performs time-division packet scheduling in such a manner that the radio channel is preferentially allocated to a mobile station having a good condition. Therefore, a smaller amount of interference noise enables communication at a higher data rate. Minimizing the interference noise is effective in improving the system performance.
A method disclosed in Japanese Unexamined Patent Application Publication No. 2002-232350 (patent document 1) has been proposed to avoid interference between base stations in the cdma2000 1xEV-DO system (former HDR system). The proposed method uses a direction-variable antenna to narrow the aperture of the radio-wave beam transmitted by each base station and to transmit the radio waves in such a direction that strong interference will not occur.
FIG. 1 shows a radio-wave transmission pattern with a conventional technology.
The figure shows that three-sector base stations 1-1 to 1-7 transmit radio-wave beams in a common frequency band of f1. Shaded portions represent the radio-wave beams, which are transmitted in such directions that interference will not occur. The beam transmission directions are switched at predetermined times so that all directions are covered.
Patent Document 1
Japanese Unexamined Patent Application Publication No. 2002-232350
Non-patent Document
The 3rd Generation Partnership Project 2 (3GPP2) Specifications (online), The 3rd Generation Partnership Project 2 (found on Nov. 9, 2004), at http://www.3gpp2.org/public_html/specs/index.cfm/